Like Secrets of the Surface World is for well, the surface world, this book is all about the Hollow Earth. Create native characters from all over (under) the Earth.
Chapter 1 again deals with Characters. At this point you know how this all set up. New Archetypes include Barbarians, Beastmen, Guardians, Healers, Mystics, Natives, Outcast and Warriors. One thing should be pretty obvious now, not only can you use this for a Pellucidar-like game, but it sets up a Barsoom game nicely or even a Conan/Hyborean Age game. A Pulp game in a Pulp setting, how nice is that! There are some new motivations, and plenty of new talents. There are also some new flaws. This book feels more like a true supplement rather than a book of "left-overs"; some thought and research went into this. I was reading through it all and mentally substituting things I had read from Edgar Rice Burroughs or Robert E. Howard. That's a good sign. Plus you can mix and match talents to create Panthermen, Hawkmen, Reptile-people (always a plus in my book) and dozens of others.
The pre-gen Archetypes are great. The Amazon Warrior makes me want to play a Xena like game now.
Chapter 2 takes us back to Supernatural Powers. We start with more details on sorcery including more modifiers. We also are given Shamanism and Alchemy which is really cool. This chapter plus it's twin in Secrets of the Surface World gives me no end of ideas.
Chapter 3 covers Natives. This is a great and fun chapter to be honest. If anyone asks me why run a game in the Hollow Earth I am directing them to this. It is an odd mix of Pulp, post-Victorian occultism and fringe science. I love it. I have seen other games take the same elements, but the assembly here is fantastic. Is it the only way to do this? No, the same elements appear in many other games (Amazons, Atlanteans, lost titans...) but here it works rather nice.
Chapter 4 Beastmen covers the others living in the Hollow Earth. Natives are largely human, beast men are something else. The usual suspects are here; Apemen, Gillmen, Lizardmen, Molemen (natch), and Panthermen (or at least a cat-like humanoid race) but there are some great newcomers like the not often seen Hawkmen (should be more Egyptian in my tastes but hey, happy to see them) and some insectmen and the new for this genre Green Men which are more plant like.
Chapter 5 covers the Hollow Earth. It includes some basics (healing, getting out) but mostly devoted to various locations. Atlantis for example is here, as is El Dorado (the City of Gold), Shangri-La, and Blood-Bay where the Pirates hang out. That is enough to keep you going for a while really.
Chapter 6 adds a more monsters to the Bestiary. There are more dinosaurs here (always welcomed!). There are prehistoric reptiles that are not dinosaurs, such as the Archelon and the Plesiosaurus among others. The science geek in me appreciates the separation. We also get a great collection of prehistoric mammals. Giant insects, giant apes, and other creatures fill this section. There is even a guide for creating your own creatures. Which is good, because the one monster I wanted wasn't there. The book has plenty of pictures of Dimetrodon, but no stats. I might have to make my own now.
We end with a sample adventure, Fate of Atlantis and an Index.
There is so much here that any half-decent GM could find hours and hours worth of game materials for their own Hollow Earth games.